Tractor Shed Design (Renamed)

Max sawdust · August 19, 2006, 06:25:32 PM

I have a low 4/12 roof pitch with an 18' span a King Post and a birdsmouth to post truss. (Yes the tie beam will be splined Jim's way ;D) My question is what do you think would be better, Struts from the kingpost to rafters or adding two queen posts ??? With the low roof pitch my gut says to make two queen posts ???

Your thoughts?

Max

Raphael · August 19, 2006, 07:22:04 PM

Sounds like a pair of queens would be more effective... Just so far out you'll be able to reach with struts.

Jim_Rogers · August 21, 2006, 09:57:20 AM

You can use both....

It really depends on the loads....

But you haven't really given us all the information needed in order for me to visualize the truss assembly.

If the rafters have a bird's mouth joint to the top of a post and you're using a king post between the rafters then how low below the birds mouth joint is the tie beam?

Do the rafters over hang the post? if so how much?

I have this:

If the rafter ends at the outside of the post then this birds mouth joint may need some redesign work as the birds mouth should be in the bottom 1/3 of the rafter.

Here in my drawing it is in the top 1/3 and that wouldn't be correct.

The distance between the outsides of the posts is 18' and the rafters are at a 4/12 pitch.

Jim Rogers

mark davidson · August 21, 2006, 03:04:49 PM

Jim, that drawing program of yours rocks!

bartfc · August 21, 2006, 05:11:05 PM

Maybe I missed something, but could you tell me "Jim's Way" of connecting the tie beam? THANKS!!

Jim_Rogers · August 21, 2006, 05:33:12 PM

Bartfc:
check out this thread for more details:
click here for link to opinions on king post variations story

Jim Rogers

Max sawdust · August 22, 2006, 07:25:44 AM

Quote from: Jim_Rogers on August 21, 2006, 09:57:20 AM
You can use both....

It really depends on the loads....

But you haven't really given us all the information needed in order for me to visualize the truss assembly.

If the rafters have a bird's mouth joint to the top of a post and you're using a king post between the rafters then how low below the birds mouth joint is the tie beam?

Do the rafters over hang the post? if so how much?

I have this:

If the rafter ends at the outside of the post then this birds mouth joint may need some redesign work as the birds mouth should be in the bottom 1/3 of the rafter.

Here in my drawing it is in the top 1/3 and that wouldn't be correct.

The distance between the outsides of the posts is 18' and the rafters are at a 4/12 pitch.

Jim Rogers

Sorry guys, away on business yesterday. Wow Jim that program is cool.
1. Loads: No live loads no second floor, only snow load.
2. Tie beam to post: I choose the birdsmouth to post connection to facilitate a full bent raising. I can bring the tie beam into the post "anywhere". Currently my thinking is very near to the post top for strength something like 12" below the birdsmouth to post connection.
3. Rafters end at the post top.
4. Redesign of the joint: Yes I immediately noticed the top of the birdsmouth was way too thin, I used the section of Chapel's book on Birdsmouth to post connection to redesign the joint. He specifically addresses low pitch roofs (at least in my book :D) First I added a one inch shoulder to the post top, secondly I increased the "beak" of the birdsmouth at outside of the post to two inches. This effectively brought the birdsmouth down from the top third of the rafter closer to the bottom third.
See pic (of birdsmouth template on post):

5. Yes the span is 18' outside of post to outside of post and the pitch is 4/12

The material is approximately 6"x8" Red Pine.

Max

Jim_Rogers · August 22, 2006, 11:14:08 AM

Ok, so I drew what you said, and added a 4x6 strut to the king post up 4" from the bottom of the king post and I forgot to cut the tie beam and extend the king post down to make it an interrupted tie.
But anyhow, I projected the strut up to the rafter at a 45° angle and it intersects the rafter about 1/3 the way down the rafter.
Knowing this, I then created a point that was 1/3 the way up from the outside of the building, or end of rafter. And placed a 4x6 queen post at that location.
Also all my other timbers are 8x8 in this drawing.

If it was me, I'd use both as now the rafter is supported at each end, and at 1/3 points. This will reduce the thrust at the top of the post and should prevent the rafter from pushing the post out above the interrupted tie.

I would use a half dovetail joint from the ties to the post with a through tenon as Steve draws it and a spline from the tie through the king to the other tie, as we have discussed in the other thread.

With these timbers assembled like this, I don't know if I'd call it a truss or not, maybe just a bent.

Jim Rogers

Jim_Rogers · August 22, 2006, 11:39:09 AM

Here it is with the interrupted tie beam and spline joint....

And here is a close up of the spline through the king post......

Just some of my ideas....

Jim Rogers

Max sawdust · August 22, 2006, 09:44:51 PM

Thanks Jim

That is one pretty bent. (Even though the roof pitch is so pitifully low :

)

Sure like that software of yours, I will have to try the free 2D version.

I started cutting the king posts today. If the joinery is not too embarrassing I will post pictures :

Sure seems like the struts are overkill for a tractor shed, but I will cut them, because this is all about learning

Got a lot to learn, have lots of respect for the very few who can design and cut a clean frame.

FYI I am using Aspen for the king posts. Interesting stuff, planes and chisels like magic (if you follow the grain), just need to knife cut your lines REAL deep because it wants to chip ;)

Jim_Rogers · August 23, 2006, 10:40:52 AM

Max:
You never mentioned or I don't remember what the snow load for your area is.
You may be able to get away from using the struts and the queen post if the rafter will support the load.
It really depends on spacing and roofing type, and other factors....

Jim Rogers

Max sawdust · August 24, 2006, 02:46:01 PM

Jim,
The snow load is 50PSF.
The roof will have Purlins. 1" T&G boards, tar paper and asphalt shingles will make up the roof.

Jim_Rogers · August 24, 2006, 07:30:05 PM

Ok, so what's the spacing between bents?
(I hope you didn't post that before, and I missed it......)

Jim Rogers

Max sawdust · August 25, 2006, 05:39:27 AM

Jim,
Bent spacing is 12' Total structure is just two bents. (tractor shed)
No second floor or loft so no live load.
Thanks Max

Jim_Rogers · August 25, 2006, 10:27:58 AM

Ok so lets look at the loads each rafter will have to hold up....

The span is 18' so the run (half the span) is 9'.
The width between bents is 12' so the amount each rafter will hold up is 6'.
So, 9'x6' is 54 sqft. Now the snow load is 50 lbs per sqft, and the dead load of the roof, boards, tar paper and shingles could be around 10 to 15 lbs per sqft. Let's say 15 to make sure we have rafters large enough, so total or combined load is 65 lbs per square feet.

Next 65 lbs x 54 sqft is 3510 lbs.....

Using DonP's calculator we now know the first number needed.

And filling out the rest of the calculator with the known numbers we see that the rafter fails in bending.

Of course this is without figuring the support by the queen post and strut.

Here is my view of the frame:

If we reduce the area each rafter is holding up by adding the strut and the queen the numbers all change.

Now the distance between the center of the queen post and the center of the strut is 3' 7 1 1/16" or 43 1/16" or 43.0625" as shown here:

So if we go back to DonP's calculator after re figuring the new reduced run the load changes to 1400 lbs. and the rafter passes on all three tests....

We should look at the size and spacing of the purlins to make sure they will pass.....

Max, what was your intended size and spacing of the purlins?

In the drawing above I made them 6x6....

Jim Rogers

Jim_Rogers · August 25, 2006, 11:08:14 AM

For fun, I ran the numbers on the 6x6 purlin at 2' spacing using red pine, and they failed.
Moving them to 16" and they passed, but just barely.
I'd make them 6x7 to be sure.....

Jim Rogers

Max sawdust · August 25, 2006, 09:07:07 PM

Jim,
I could use 6x8's Red Pine for the Purlins. I am astonished by what you came up with

Wow that many Purlins?
I was unsure of Purlin spacing. :

There is more to the engineering than I currently comprehend ;)
Your assistance is deeply appreciated

Have you considered teaching an on line class? Many people can not afford the time away from home or the costs associated with travel to a "hands on" class.

Max

Raphael · August 26, 2006, 01:13:41 AM

Might consider switching wood species for the purlins so they don't wind up looking too large and crowded.

Jim_Rogers · August 26, 2006, 07:53:56 AM

Max:
Unless someone else (like DonP) runs the numbers and proves me wrong (which is possible, and I'll yield to the numbers) this is the way I see it, based on the facts you've provided.....
Also you'll have to do something at the ridge for the boards to attach to, up there.
I didn't draw one in as I wasn't sure how you'd do it.
I would not cut joints into the rafters for the purlins to connect as this will weaken them, and I wasn't sure how much gable overhang you would have. So, yes, 6x8 if you're going to have a foot or more of gable overhang and yes you could change types of wood to a stronger one.....
You can build it with less or smaller pieces but be prepared to shovel all the snow off your roof after every storm, every winter......

Jim Rogers

Max sawdust · August 26, 2006, 08:10:19 AM

Jim,
I was considering using a 6x8 ridge beam, then I could run a strut from the king post to ridge beam to help stiffen up the roof. Also their will be bracing from the top plate to the post and the top plate is part of the roof system. (IF my purlins are cut into the primary rafters.)

I really need my purlins to be flush with the primary rafters. I can house the dovetail joint to reduce material used and increase strength.

By having the purlins on top of the primary rafters I would have to shorten my posts by six inches. So I would have to shorten my posts by six inches which is undesirable. The reason I can not add height is because the building tucks up under the gable overhang of another building.

Is the problem the Purlins or the primary rafters?

Shoveling snow is not an option.

Thanks again
Max

Max sawdust · August 26, 2006, 08:12:46 AM

Raphael,
Yes Red Oak would be nice, but I am trying to save my standing inventory for more "grand" projects than a tractor shed. :D

Max

Max sawdust · August 26, 2006, 08:24:01 AM

I for the first time went to the Don P calculator :

Wow what a great tool now that Jim got me started.

Jim,
I think you used #2 grade for the Red Pine numbers in the calculator. My beams are all heart center few tight knots in them So they are "Select"

By choosing select instead of #2 the first calculation now passes.

I still do not understand, the calculation example you gave seems to be for the Primary rafters not the Purlins. How can you determine Purlin size and spacing based on the example?
Is this correct thinking?
Here is what my thinking for each side of the roof is. 9'x12'=108sq x 65lbsq load=7020lb divide this by 4 (purlins on 2ft cetners)

Load on Beam(pounds) 1755
Span of Beam (inches) 144
Width of Beam 6
Depth of Beam 7
Maximum Allowable Fiberstress in Bending (PSI) 1050
Modulus of Elasticity (million PSI) 1.1
Maximum Allowable Horizontal Shear (PSI) 65

All Pass

I will play more now that you got me started ;)
Max

Raphael · August 26, 2006, 01:39:16 PM

I don't think Jim ran the numbers for a 6x7 2'OC...

My calculations say you'll need a minimum 4 purlins on each roof plane and one at the ridge...
That's just squeeking by using the #2 numbers... I'd be leary of using select structural numbers unless they were graded by an expert. If I felt absolutley certain they were select structural I'd call them #1 just to be on the safe side.

There is a flaw in your approach to the math (actually two). Presumably you'll have a purlin right out at the eves, this purlin actually carries half the total load of it's neighbor (ignoring any overhang). Your rafter is going to be ~9.5' long so If you've got a purlin at the eves and a purlin near the ridge (say the 9' mark) then the remaining 2 purlins wind up being 3' OC so they carry 36sq.ft. or 2340lbs. The other flaw is minor, the value for span is actually the clear span. Each purlin is supported over the rafter for 6" on each end so the clear span is 132" or 133" if you work to a 5½" perfect timber. Using #2 values this would require Red Maple or stronger (Northern Red Oak, Beech|Birch|Hickory, & Doug.Fir) timbers.

Hope this helps more than it confuses...

Jim_Rogers · August 26, 2006, 05:20:41 PM

Max:
Keeping your purlins flush with the top of the rafters adds a lot more to consider.
I can't go into it a lot right now as I have to leave to go home for the weekend.
But I'll discuss it further with you on Monday......

Jim Rogers

Max sawdust · August 26, 2006, 06:36:31 PM

Raphael,
Do not know if this makes sense but the top plate, which will be joined to the posts will support part of the roof load. I plan on having the top plate mounted high on the post and plane mill it to match the roof line. Also, I have no overhangs whatsoever. I plan on having a ridge beam, that will have a strut back to the king post. (Most of these idea's come from Chappell's book)

Most of what you said makes sense, I just need to read it 20 or 30 more times :

:D

Thanks,

Jim,
Thanks for the response, I explore various housing methods to maintain strength in the purlins and not take too much from the principal rafters. Enjoy your trip!

My head hurts but the learning is fun ;D

Respectfully,
Max